First, I'd like to establish which customers we are attempting to retain. There are some customers that will go to Microsoft teams because of Microsoft loyalty and certain core functionality. I'm not admitting defeat; I am simply trying to establish the type of consumer we want to retain. If we can decide on a set of customers that have requirements in common, we can atttempt to retain them. For the sake of the question, let's say we want to retain our startup and small company customers: any company with a size <= 2000 employees. We want to focus on them.
Secondly, I'd like to identify the key use cases that are causing startups to leave Slack. One key problem I see is that Microsoft has capitalized on overall enterprise software (sharepoint, oneNote,etc.). Teams integrating with this suite of services is a natural complement which may appeal to companies. Another key use case I imagine being important is pricing; both Slack and Microsoft are similar in pricing I believe. If this is true then the same price+ integration with microsoft tools is ample reason to swtich. For the sake of the question, I am sticking with these two key use cases that are drawing customers away from Slack.
Third, I'd like to suggest a feature and a strategy to help mitigate the loss of customers. The feature is an extension to make file sharing and saving easier. I think startups generally try to stay lean and building in an easy file sharing and saving extension would greatly improve the path between saving a file and sending it out. A strategy I woud suggest is to manipulating pricing strategies. A productivity tool is hard to switch from when it contains everything about your company and the information. To raise the switching cost, I'd like to institute promotional pricing that can last up to a year (pending cost-benefit analysis works). This period of decreased prcing can get more startup and small company customers to on board. So, when the price increases again, the switching cost will not be worth the time.
To sum it up: implement a new file sharing/saving extension to make file sharing easier, and heavily pursue promotional pricing to get startup and small company customers on boarded.